Lignin by-products of the black liquor residue of a kraft pulping process have been variously employed as additives in other chemical compositions. Sulfonated lignins are used as dispersants in textile dyestuff compositions and pigment printing pastes. Lignin by-products have also found use as grinding aids in oil well drilling operations, and as emulsifiers in other chemical compositions. Various methods of recovery, purification, and modification of lignin by-products as water-soluble sulfonated and sulfomethylated salts for use as additives in dyestuff compositions are described and disclosed in the following U.S. Pat. Nos. 2,525,433; Adler et al. 2,680,113; 2,690,973; Keirstead et al. 3,094,515; Baisdell 3,158,520; 3,503,762; Detroit 3,726,850; Hintz 3,769,272; Falkehag et al. 3,841,887; Dilling et al. 4,001,202; Dilling 4,131,564; Lin 4,184,845; Lin 4,308,203; and Dilling et al. 4,355,996.
Generally, the lignin material of a kraft pulping process employed in paper making is recovered from the black liquor by lowering the pH of the liquor with carbon dioxide from an initial pH of about 13 to a pH of around 9.5 to precipitate the lignin material from the black liquor. The pH of the precipitated lignin slurry which may be modified, as by sulfonation, if desired, is further lowered with sulfuric acid to a pH of about 1.5 to 3, at which point the solid lignin is washed with water to remove inorganic salts and impurities therefrom. The purified lignins are then employed as additives in specified chemical compositions.
In U.S. Pat. No. 4,470,876, Beaupre' et al. teach a process for the treatment of kraft black liquor which comprises acidifying the black liquor with sulfuric acid "where the pH of the mixture is maintained at most about 5, preferably below about 4.5" (col. 2, lines 30-31), prior to separation of the lignin therefrom.
Recent attempts have been made to use such purified lignin materials in water-based carbon black printing compositions both as a grinding aid and as an adhesive to bind the carbon black to the substrate in printing operations. To render the purified lignins water-soluble, an organic amine, such as monoethanolamine (MEA), must be added to an aqueous lignin slurry to form a water-soluble lignin amine salt for addition to the printing composition. However, this proposed use of lignins in such water-based printing compositions has been rendered impractical because the lignin exhibits poor solubility and poor stability in aqueous solution with MEA. In particular, it has been found that the poor water solubility of the amine lignin salts prepared as described results in sludging and viscosity variations with consequent reduced effectiveness of the formulated printing compositions.